U.S. patent application number 13/087524 was filed with the patent office on 2012-01-26 for antenna device and communication device.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Shohei ISHIKAWA, Teruhisa NINOMIYA.
Application Number | 20120019427 13/087524 |
Document ID | / |
Family ID | 44675414 |
Filed Date | 2012-01-26 |
United States Patent
Application |
20120019427 |
Kind Code |
A1 |
ISHIKAWA; Shohei ; et
al. |
January 26, 2012 |
ANTENNA DEVICE AND COMMUNICATION DEVICE
Abstract
An antenna device includes a helical antenna that radiates a
circularly polarized radio wave in a direction of an axis S, in
which the helical antenna is formed with linear conducting bodies
each helically winding around the predetermined axis and adjacent
linear conducting bodies are spaced a distance P apart. The antenna
device further includes a director that is on the axis S, along
which a radio wave is radiated from the helical antenna, at a
position a distance X away from the helical antenna, the distance X
being substantially equal to the distance P.
Inventors: |
ISHIKAWA; Shohei; (Kawasaki,
JP) ; NINOMIYA; Teruhisa; (Kawasaki, JP) |
Assignee: |
FUJITSU LIMITED
Kawasaki-shi
JP
|
Family ID: |
44675414 |
Appl. No.: |
13/087524 |
Filed: |
April 15, 2011 |
Current U.S.
Class: |
343/833 |
Current CPC
Class: |
H01Q 11/08 20130101;
H01Q 19/28 20130101; H01Q 1/2216 20130101 |
Class at
Publication: |
343/833 |
International
Class: |
H01Q 19/02 20060101
H01Q019/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 20, 2010 |
JP |
2010-163215 |
Claims
1. An antenna device comprising: an antenna that radiates a
circularly polarized radio wave in a direction of a predetermined
axis, wherein the antenna is formed with linear conducting bodies
each helically winding around the predetermined axis, and adjacent
linear conducting bodies are spaced a first distance apart; and a
director that is on the axis, along which a radio wave is radiated
from the antenna, at a position a second distance away from the
antenna, wherein the second distance is substantially equal to the
first distance.
2. The antenna device according to claim 1, wherein the director
includes a plurality of conductive elements, each conductive
element being on the axis along which a radio wave is radiated from
the antenna, and a distance between the antenna and a conductive
element closest to the antenna and a distance between adjacent
conductive elements are substantially equal to the second
distance.
3. The antenna device according to claim 2, wherein the number of
the conductive elements is an even number.
4. The antenna device according to claim 2, wherein the conductive
elements are in a cross shape, viewed from a plate perpendicular to
the axis along which a radio wave is radiated from the antenna.
5. The antenna device according to claim 2, wherein the conductive
elements are in a hooked cross shape, viewed from a plate
perpendicular to the axis along which a radio wave is radiated from
the antenna.
6. The antenna device according to claim 1, wherein the first
distance and the second distance are values not higher than
0.15.lamda. but not lower than 0.11.lamda., where .lamda. is an
equivalent wavelength of a radio wave radiated from the
antenna.
7. The antenna device according to claim 6, wherein the number of
the conductive elements is an even number.
8. The antenna device according to claim 6, wherein the conductive
elements are in a cross shape, viewed from a plate perpendicular to
the axis along which a radio wave is radiated from the antenna.
9. The antenna device according to claim 6, wherein the conductive
elements are in a hooked cross shape, viewed from a plate
perpendicular to the axis along which a radio wave is radiated from
the antenna.
10. A communication device for radiating a radio wave toward a tag
to make a communication with the tag, the communication device
comprising: an antenna that radiates a circularly polarized radio
wave in a direction of a predetermined axis, wherein the antenna is
formed with linear conducting bodies each helically winding around
the predetermined axis, and adjacent linear conducting bodies are
spaced a first distance apart; and a director that is on the axis,
along which a radio wave is radiated from the antenna, at a
position a second distance away from the antenna, wherein the
second distance is substantially equal to the first distance.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based upon and claims the benefit of
priority of the prior Japanese Patent Application No. 2010-163215,
filed on Jul. 20, 2010, the entire contents of which are
incorporated herein by reference.
FIELD
[0002] The embodiment discussed herein is directed to an antenna
device and a communication device.
BACKGROUND
[0003] Radio frequency identification (RFID), which is a
contactless identification technology, has been used more and more,
recently. The RFID technology enables data transfer between a
communication device, such as a reader/writer, and a target
recording medium for identification, such as an integrated circuit
(IC) tag, using radio wave.
[0004] An antenna device is included in an RFID communication
device and the antenna device is, for example, a circular polarized
antenna that radiates a circularly polarized wave. A typical
circular polarized antenna is, for example, a helical antenna. A
helical antenna is an antenna formed with a linear conducting body
helically winding around an axis and radiates a circularly
polarized radio wave in a direction of the axis that is at the
center of the spiral.
[0005] The directivity of a stand-alone helical antenna is low.
When a helical antenna with a low directivity targets a certain IC
tag for identification, another IC tag near the target IC tag may
accidentally be in a coverage area of a radio wave radiated from
the helical antenna.
[0006] A well-known antenna device that increases the antenna
directivity further includes a director in a direction of a radio
wave radiated from a helical antenna. By the director in a
direction of a radio wave radiated from the helical antenna, the
antenna device increases the antenna directivity.
[0007] However, a conventional antenna device that has a direction
in a radio wave radiated from a helical antenna is too large.
[0008] Patent Document 1: Japanese Laid-open Patent Publication No.
2008-123231
[0009] Patent Document 2: Japanese Laid-open Patent Publication No.
04-216204
SUMMARY
[0010] According to an aspect of an embodiment of the invention, an
antenna device includes an antenna that radiates a circularly
polarized radio wave in a direction of a predetermined axis,
wherein the antenna is formed with linear conducting bodies each
helically winding around the predetermined axis, and adjacent
linear conducting bodies are spaced a first distance apart; and a
director that is on the axis, along which a radio wave is radiated
from the antenna, at a position a second distance away from the
antenna, wherein the second distance is substantially equal to the
first distance.
[0011] The object and advantages of the embodiment will be realized
and attained by means of the elements and combinations particularly
pointed out in the claims.
[0012] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are not restrictive of the embodiment, as
claimed.
BRIEF DESCRIPTION OF DRAWINGS
[0013] FIG. 1 is a block diagram of a reader/writer that includes
an antenna device according to an embodiment;
[0014] FIG. 2 is a perspective view of the antenna device according
to the present embodiment;
[0015] FIG. 3 is a side view of the antenna device illustrated in
FIG. 2;
[0016] FIG. 4 is an external view of an example of the dielectric
body when .lamda.=220 mm;
[0017] FIG. 5 is a schematic diagram that illustrates how a radio
wave is radiated from a stand-alone helical antenna;
[0018] FIG. 6 is a schematic diagram that illustrates how a radio
wave is radiated from the antenna device according to the present
embodiment;
[0019] FIG. 7 is a graph of the relation between a distance X and a
distance Y and a front-back ratio where distance
P=0.125.lamda.;
[0020] FIG. 8 is a graph of the relation between the distance X and
the distance Y and a half-width where the distance
P=0.125.lamda.;
[0021] FIG. 9 is a graph of the relation between the distance X and
the distance Y and a gain where the distance P=0.125.lamda.;
[0022] FIG. 10 is a graph of the relation between the distance X
and the distance Y and the front-back ratio where distance
P=0.114.lamda.;
[0023] FIG. 11 is a graph of the relation between the distance X
and the distance Y and the half-width where the distance
P=0.114.lamda.;
[0024] FIG. 12 is a graph of the relation between the distance X
and the distance Y and the gain where the distance
P=0.114.lamda.;
[0025] FIG. 13 is an external view of an example of the conductive
element;
[0026] FIG. 14 is a graph of change in the front-back ratio
depending on the number of the conductive elements;
[0027] FIG. 15 is a perspective view of a first modification of the
conductive elements;
[0028] FIG. 16 is a perspective view of a second modification of
the conductive elements; and
[0029] FIG. 17 is a perspective view of a third modification of the
conductive elements.
DESCRIPTION OF EMBODIMENT
[0030] A preferred embodiment of the present invention will be
explained with reference to accompanying drawings. The antenna
device disclosed herein is included in an RFID communication device
or a reader/writer. It is noted that the antenna device and the
communication device of the present invention is not limited to the
following embodiment.
[0031] A reader/writer that includes an antenna device according to
an embodiment is described below. FIG. 1 is a block diagram of a
reader/writer 100 according to the embodiment. In the example
illustrated in FIG. 1, there is an IC tag 200 near the
reader/writer 100. A broken line 110a of FIG. 1 indicates a
radio-wave coverage area. As illustrated in FIG. 1, the
reader/writer 100 includes an antenna device 110, an input unit
120, an output unit 130, and a control circuit 140.
[0032] The antenna device 110 is included in or mounted on the
reader/writer 100. The antenna device 110 radiates a radio wave
using a power supplied from the control circuit 140. The antenna
device 110 receives a radio wave coming and outputs the received
radio wave to the control circuit 140. The configuration of the
antenna device 110 will be described in detail later.
[0033] The input unit 120 is an input unit that is used to input
various information. The input unit 120 is, for example, an
operation button to switch on a communication function of the
reader/writer 100. The output unit 130 is an output unit that
outputs various pieces of information. The output unit 130 is, for
example, a liquid crystal display and a speaker that output a
communication result made by the reader/writer 100.
[0034] The control circuit 140 controls the reader/writer 100. The
control circuit 140 supplies a power to the antenna device 110. The
control circuit 140 makes a communication with a recording medium
or the IC tag 200 using a radio wave radiated from the antenna
device 110. For example, the control circuit 140 writes data to the
IC tag 200 using a radio wave radiated from the antenna device 110
and reads data from the IC tag 200 using a radio wave coming.
[0035] The configuration of the antenna device 110 illustrated in
FIG. 1 is described below with reference to FIGS. 2 and 3. FIG. 2
is a perspective view of the antenna device 110 according to the
present embodiment. FIG. 3 is a side view of the antenna device 110
illustrated in FIG. 2. As illustrated in FIGS. 2 and 3, the antenna
device 110 includes a helical antenna 111 and a director 112.
Although, in the example illustrated in FIGS. 2 and 3, the director
112 includes four conductive elements 112a, the number of the
conductive elements 112a can be less than or more than four. The
helical antenna 111 and the director 112 are fixed on a casing (not
illustrated).
[0036] The helical antenna 111 is a circular polarized antenna that
radiates a circularly polarized radio wave. The helical antenna 111
used in the present embodiment is a four-line helical antenna that
is formed with four linear conducting bodies 111a to 111d each
helically winding around a central axis S of a cylindrical
dielectric body 113 on a circumference of the cylindrical
dielectric body 113 spaced a distance P apart. The helical antenna
111 radiates a circularly polarized radio wave in the direction of
the central axis S of the cylindrical dielectric body 113.
[0037] One end of each of the linear conducting bodies 111a to 111d
is connected to a cross-shaped power-supply conducting body 115.
The power-supply conducting body 115 has a power-supply point 114
that receives a power from the control circuit 140 at the center
thereof. The other end of each of the linear conducting bodies 111a
to 111d is connected to a cross-shaped radiation conducting body
116 that radiates a circularly polarized radio wave in accordance
with a power received at the power-supply point 114 from the
control circuit 140.
[0038] The distance P among the linear conducting bodies 111a to
111d of the helical antenna 111 is a value not higher than
0.15.lamda. but not lower than 0.11.lamda., where .lamda. is the
equivalent wavelength of a current circularly polarized radio wave
and, more particularly, the distance P is a value close to, for
example, 0.125.lamda.. The equivalent wavelength .lamda. is
expressed by .lamda.=.lamda.0/( .epsilon.eff), where .lamda.0 is
the wavelength of a current circularly polarized radio wave in
vacuum and .epsilon.eff is the effective dielectric constant of the
dielectric body 113 with respect to the helical antenna 111.
[0039] An entire length L and a diameter D of the cylindrical
dielectric body 113 attached with the helically winding linear
conducting bodies 111a to 111d are about 0.25.lamda.. FIG. 4 is an
external view of an example of the dielectric body 113 when
.lamda.=220 mm. As illustrated in FIG. 4, when .lamda.=220 mm, the
entire length L of the cylindrical dielectric body 113 is 55 mm and
the diameter D is 58 mm. That is, the entire length L and the
diameter D of the cylindrical dielectric body 113 are about
0.25.lamda..
[0040] A manner of setting the distance P, the entire length L, and
the diameter D of the linear conducting bodies 111a to 111d is
described in J. D. Kraus, R. J. Marhefka "Antennas" 3rd Ed.,
Chapter 8, pp. 292, McGRAW-HILL; therefore, the manner is not
described in details herein.
[0041] The director 112 is on the central axis S of the dielectric
body 113, from which a radio wave is radiated from the helical
antenna ill, at a position a distance X away from the helical
antenna 111. The director 112 includes the four conductive elements
112a aligned on the central axis S of the dielectric body 113.
Arrangement of the conductive elements 112a is described with
reference to an example illustrated in FIG. 3.
[0042] As illustrated in FIG. 3, the four conductive elements 112a
are on the central axis S of the cylindrical dielectric body 113
within an area on the radiation conducting body 116 side of the
helical antenna 111. One conductive element 112a that is closest to
the helical antenna 111 among the four conductive elements 112a is
at a position the distance X away from the helical antenna 111. The
adjacent conductive elements 112a are spaced a distance Y
apart.
[0043] The reason why the director 112 is aligned on the central
axis S of the dielectric body 113 along which a radio wave is
radiated from the helical antenna 111 is described below. FIG. 5 is
a schematic diagram that illustrates how a radio wave is radiated
from the stand-alone helical antenna 111. FIG. 6 is a schematic
diagram that illustrates how a radio wave is radiated from the
antenna device 110 according to the present embodiment. The broken
lines 110a of FIGS. 5 and 6 indicate the radio-wave coverage
areas.
[0044] As illustrated in FIG. 5, if no director 112 is present on
the central axis S along which a radio wave is radiated from the
helical antenna 111, i.e., the helical antenna 111 is a stand-alone
device, the coverage area of the radio wave coming from the helical
antenna 111 is relatively wide. In other words, if the helical
antenna 111 is a stand-alone device, the antenna directivity is
low. In this case, there is a high possibility that a wrong IC tag
near the target IC tag 200 for identification is within the
coverage area of the radio wave radiated from the helical antenna
111.
[0045] In contrast, as illustrated in FIG. 6, if the director 112
is on the central axis S along which a radio wave is radiated from
the helical antenna 111, the director 112 guides the radio wave
radiated from the helical antenna 111 in a direction from the
helical antenna 111 to the director 112. In other words, the
director 112 works as a radio-wave guide. The director 112 narrows
the coverage area of the radio wave coming from the helical antenna
111 as it is compared with the situation illustrated in FIG. 5.
Moreover, the director 112 extends the coverage area of the radio
wave coming from the helical antenna 111 along the central axis S
of the dielectric body 113 in the direction from the helical
antenna 111 to the director 112. As a result, the directivity of
the antenna device 110 increases in the direction from the helical
antenna 111 to the director 112.
[0046] As described above, the reason why the director 112 is
arranged on the central axis S along which a radio wave is radiated
from the helical antenna 111 is to increase the directivity of the
antenna device 110 in the direction from the helical antenna 111 to
the director 112. With this configuration, even if there is another
IC tag near the target IC tag 200 for identification, any IC tag
other than the target IC tag 200 is likely to keep out of the
coverage area of the radio wave coming from the helical antenna
111.
[0047] Moreover, in the present embodiment, the distance X between
the helical antenna 111 and the conductive element 112a that is
closest to the helical antenna 111 and the distance Y between the
adjacent conductive elements 112a are substantially equal to the
distance P among the linear conducting bodies 111a to 111d. That
is, the distance X between the helical antenna 111 and the
conductive element 112a that is closest to the helical antenna 111
and the distance Y between the adjacent conductive elements 112a
are values not higher than 0.15.lamda. but not lower than
0.11.lamda.. The distance X and the distance Y are, preferably,
values not higher than 0.13.lamda. but not lower than 0.12.lamda..
The distance X and the distance Y are, more preferably,
0.125.lamda..
[0048] The reason why the distance X and the distance Y are set to
values substantially equal to the distance P among the linear
conducting bodies 111a to 111d or values not higher than
0.15.lamda. but not lower than 0.11.lamda. is described below. Some
antennas called Yagi-Uda antennas have a dipole antenna and a
director apart a distance about 0.25.lamda. from each other. When
Yagi-Uda antennas are taken into consideration, it may be said that
the distance X and the distance Y of the present embodiment are
preferably about 0.25.lamda.. In a Yagi-Uda antenna, the distance
between the dipole antenna and the director is set to a value about
0.25.lamda. in order to increase the front-back ratio. The
front-back ratio is a ratio between an electric field of a radio
wave radiated in a direction from the antenna to the director and
an electric field of a radio wave radiated in a direction
180.degree. opposite to the direction. As the front-back ratio
increases, the directivity of the antenna device increases in the
direction from the antenna to the director. A typical handy
reader/writer that includes a helical antenna inside its casing has
a low directivity and may read not only a target tag but also a
wrong tag near the target tag. The technique used in the present
device is not a technique for modifying a reader/writer and is
different from the technique used in a Yagi antenna in which an
external director is at a position a quarter wavelength away from
an antenna. The present antenna device is characterized in an
external director present at a position a certain distance away
from a helical antenna that is included in the present
reader/writer, in which the certain distance is equal to a distance
between conductive elements of the helical antenna.
[0049] If the distance X and the distance Y are about 0.25.lamda.
and the equivalent wavelength .lamda. of the current radio wave is
relatively high, the antenna device 110 becomes large and it is
difficult to mount the large antenna device 110 on the
reader/writer 100. If the current radio wave is a typical radio
wave used in RFID, for example, an about-950-MHz UHF radio wave,
the distance X and the distance Y become 7.89 cm and it is
difficult to mount the large antenna device 110 on the handy and
small reader/writer 100.
[0050] As these facts are taken into consideration, the inventors
studied a way of increasing, keeping the distance X and the
distance Y lower than 0.25.lamda., the front-back ratio to a high
level equal to the front-back ratio corresponding to the distance X
and the distance Y being 0.25.lamda.. Consequently, the inventors
found that, if the distance X and the distance Y are substantially
equal to the distance P among the linear conducting bodies 111a to
111d, i.e., values not higher than 0.15.lamda. but not lower than
0.11.lamda., the front-back ratio is increased to a level higher
than the front-back ratio corresponding to the distance X and the
distance Y being about 0.25.lamda..
[0051] The above finding is described below with reference to
examples illustrated in FIGS. 7 to 12. FIG. 7 is a graph of the
relation between the distance X and the distance Y and the
front-back ratio where the distance P=0.125.lamda.. The horizontal
axis of the graph of FIG. 7 is the distance X and the distance Y;
the vertical axis of the graph of FIG. 7 is the front-back ratio
[dB]. The effective dielectric constant .epsilon.eff of the
dielectric body 113 with respect to the helical antenna 111 is,
herein, "1"; and the equivalent wavelength .lamda. is equal to the
wavelength .lamda.0 of the current radio wave in vacuum. As
illustrated in FIG. 7, if the distance X and the distance Y are
0.25.lamda., the front-back ratio is 13.16 [dB]. As the distance X
and the distance Y decrease from 0.25.lamda., the front-back ratio
decreases from 13.16 [dB], once. However, if the distance X and the
distance Y are substantially equal to the distance P, i.e., values
not higher than 0.15.lamda. but not lower than 0.11.lamda., the
front-back ratio is higher than the front-back ratio 13.16 [dB]
that is the level corresponding to the distance X and the distance
Y being 0.25.lamda.. The front-back ratio marks the highest value
15 [dB] when the distance X and the distance Y are equal to the
distance P, i.e., 0.125.lamda..
[0052] FIG. 8 is a graph of the relation between the distance X and
the distance Y and the half-width where the distance
P=0.125.lamda.. The horizontal axis of the graph of FIG. 8 is the
distance X and the distance Y; the vertical axis of the graph of
FIG. 8 is the half-width [deg]. The half-width is an angular width
at which the power of the radio wave radiated from the antenna is a
half maximum. As illustrated in FIG. 8, if the distance X and the
distance Y are substantially equal to the distance P, i.e., values
not higher than 0.15.lamda. but not lower than 0.11.lamda., the
half-width is substantially equal to the half-width corresponding
to the distance X and the distance Y being 0.25.lamda..
[0053] FIG. 9 is a graph of the relation between the distance X and
the distance Y and the gain where the distance P=0.125.lamda.. The
horizontal axis of the graph of FIG. 9 is the distance X and the
distance Y; the vertical axis of the graph of FIG. 9 is the gain
[dBi]. The gain is a ratio between the power of an antenna to be
measured and the power of the reference antenna, the powers being
measured when the same power is supplied to the antennas. As
illustrated in FIG. 9, the distance X and the distance Y are
substantially equal to the distance P, i.e., values not higher than
0.15.lamda. but not lower than 0.11.lamda., the gain is
substantially equal to the gain corresponding to the distance X and
the distance Y being 0.25.lamda..
[0054] FIG. 10 is a graph of the relation between the distance X
and the distance Y and the front-back ratio where the distance
P=0.114.lamda.. The horizontal axis of the graph of FIG. 10 is the
distance X and the distance Y; the vertical axis of the graph of
FIG. 10 is the front-back ratio [dB]. The effective dielectric
constant .epsilon.eff of the dielectric body 113 with respect to
the helical antenna 111 is, herein, a predetermined value; and the
equivalent wavelength .lamda. is shorter than the wavelength
.lamda.0 of the current radio wave in vacuum. As illustrated in
FIG. 10, if the distance X and the distance Y are 0.34.lamda., the
front-back ratio is 34.7 [dB]. As the distance X and the distance Y
decrease from 0.34.lamda., the front-back ratio decreases from 34.7
[dB], once. However, if the distance X and the distance Y are
substantially equal to the distance P, i.e., are values not higher
than 0.15.lamda. but not lower than 0.11.lamda., the front-back
ratio is higher than the front-back ratio 34.7 [dB] that is the
level corresponding to the distance X and the distance Y being
0.34.lamda.. The front-back ratio marks the highest value 38 [dB]
when the distance X and the distance Y are equal to the distance P,
i.e., 0.114.lamda..
[0055] FIG. 11 is a graph of the relation between the distance X
and the distance Y and the half-width where the distance
P=0.114.lamda.. The horizontal axis of the graph of FIG. 11 is the
distance X and the distance Y; the vertical axis of the graph of
FIG. 11 is the half-width [deg]. As illustrated in FIG. 11, if the
distance X and the distance Y are substantially equal to the
distance P, i.e., values not higher than 0.15.lamda. but not lower
than 0.11.lamda., the half-width is substantially equal to the
half-width corresponding to the distance X and the distance Y being
0.34.lamda..
[0056] FIG. 12 is a graph of the relation between the distance X
and the distance Y and the gain where the distance P=0.114.lamda..
The horizontal axis of the graph of FIG. 12 is the distance X and
the distance Y; the vertical axis of the graph of FIG. 12 is the
gain [dBi]. As illustrated in FIG. 12, the distance X and the
distance Y are substantially equal to the distance P, i.e., values
not higher than 0.15.lamda. but not lower than 0.11.lamda., the
gain is substantially equal to the gain corresponding to the
distance X and the distance Y being 0.34.lamda..
[0057] As described above, if the distance X and the distance Y are
substantially equal to the distance P among the linear conducting
bodies 111a to 111d, i.e., values not higher than 0.15.lamda. but
not lower than 0.11.lamda., the front-back ratio is higher than the
front-back ratio corresponding to the distance X and the distance Y
being about 0.25.lamda.. At the same time, the half-width and the
gain are substantially equal to the half-width and the gain each
corresponding to the distance X and the distance Y being about
0.25.lamda., respectively. Therefore, in the antenna device 110
according to the present embodiment, the distance X and the
distance Y are set to values substantially equal to the distance P
among the linear conducting bodies 111a to 111d, i.e., values not
higher than 0.15.lamda. but not lower than 0.11.lamda.. With this
configuration, compared with a Yagi-Uda antenna in which the
distance between the antenna and the director is about 0.25.lamda.,
the antenna device 110 according to the present embodiment has an
increased antenna directivity with a decreased volume.
[0058] Moreover, in the present embodiment, the four conductive
elements 112a are in a cross shape, viewed from a plane
perpendicular to the central axis S of the dielectric body 113.
FIG. 13 is an external view of an example of the conductive element
112a. As illustrated in FIG. 13, the 1-mm-thick conductive element
112a is in a cross shape with the length 120 mm and the width 5 mm,
viewed from a plane perpendicular to the central axis S of the
dielectric body 113.
[0059] As described above, in the present embodiment, the four
conductive elements 112a are in a cross shape, viewed from a plane
perpendicular to the central axis S of the dielectric body 113,
which increases the sensitivity of a radio wave with respect to a
plate perpendicular to the central axis S of the dielectric body
113.
[0060] As, in the present embodiment, the director 112 includes the
four conductive elements 112a, the number of the conductive
elements 112a is, preferably, an even number. The reason is
described with reference to FIG. 14.
[0061] FIG. 14 is a graph of change in the front-back ratio
depending on the number of the conductive elements 112a. The
horizontal axis of the graph of FIG. 14 is the number of the
conductive elements 112a; the vertical axis of the graph of FIG. 14
is the front-back ratio [dB]. As illustrated in FIG. 14, a
front-back ratio corresponding to an even number of the conductive
elements 112a is higher than a front-back ratio corresponding to an
odd number of the conductive elements 112a. For example, the
front-back ratio corresponding to two conductive elements 112a is
higher than the front-back ratios corresponding to one or three
conductive elements 112a.
[0062] As described above, if the number of the conductive elements
112a is an even number, as compared with the number being an odd
number, a higher front-back ratio is obtained. Therefore, in the
antenna device 110 according to the present embodiment, the
director 112 includes the four conductive elements 112a.
[0063] As described above, the antenna device 110 according to the
present embodiment includes the helical antenna 111 that is formed
with the four linear conducting bodies 111a to 111d each helically
winding around the central axis S of the dielectric body 113 spaced
the distance P apart. The helical antenna 111 radiates a circularly
polarized radio wave in the direction of the central axis S. The
antenna device 110 further includes the director 112 on the central
axis S, along which a radio wave is radiated from the helical
antenna 111, at a position the distance X away from the helical
antenna 111, the distance X being substantially equal to the
distance P. With this configuration, the antenna device 110 can
increase the antenna directivity to a sufficiently high level in a
direction from the helical antenna 111 to the director 112. The
antenna device 110 can especially increase the front-back ratio
that is a major index of the antenna directivity.
[0064] Moreover, in the antenna device 110 according to the present
embodiment, the director 112 includes the two or more conductive
elements 112a, each located on the central axis S along which a
radio wave is radiated from the helical antenna 111. The distance
X, which is the distance between the helical antenna 111 and the
conductive element 112a that is closest to the helical antenna 111,
and the distance Y, which is the distance between the adjacent
conductive elements 112a, are substantially equal to the distance P
among the linear conducting bodies 111a to 111d. With this
configuration, compared with a director including one conductive
element, the antenna device 110 has an increased antenna
directivity.
[0065] Furthermore, in the antenna device 110 according to the
present embodiment, the distance X and the distance Y are
substantially equal to the distance P among the linear conducting
bodies 111a to 111d, i.e., values not higher than 0.15.lamda. but
not lower than 0.11.lamda.. With this configuration, compared with
a Yagi-Uda antenna in which the distance between the antenna and
the director is about 0.25.lamda., the antenna device 110 has an
increased antenna directivity with a decreased volume.
[0066] Moreover, in the antenna device 110 according to the present
embodiment, the number of the conductive elements 112a is an even
number. With this configuration, compared with a case where the
number of the conductive elements 112a is an odd number, the
front-back ratio is increased to a higher level and the antenna
device 110 has an increased antenna directivity.
[0067] Furthermore, in the antenna device 110 according to the
present embodiment, the four conductive elements 112a are in a
cross shape, viewed from a plane perpendicular to the central axis
S along which a radio wave is radiated from the helical antenna
111. With this configuration, the antenna device 110 can
transmit/receive a circularly polarized radio wave at a high
sensitivity.
[0068] Although, in the present embodiment, the four conductive
elements 112a are in a cross shape, viewed from a plane
perpendicular to the central axis S along which a radio wave is
radiated from the helical antenna 111, the shape of the conductive
elements 112a is not limited thereto. A first modification, a
second modification, and a third modification of the shape of the
conductive elements 112a are described below. FIGS. 15 to 17 are
perspective views of a first modification, a second modification,
and a third modification of the conductive elements 112a. As
illustrated in FIGS. 15 to 17, the four conductive elements 112a
can be in a hooked cross shape, viewed from a plane perpendicular
to the central axis S from the helical antenna 111. When they are
compared with the cross-shaped conductive elements, the
hooked-cross-shaped conductive elements 112a reduces the outline of
the director 112, keeping the length of the conductive elements
unchanged. With this configuration, an antenna device having a
decreased volume can transmit/receive a circularly polarized radio
wave at a sensitivity as high as that of the antenna device having
cross-shaped conductive elements.
[0069] Although, in the present embodiment, the antenna device
includes, as an example of the circular polarized antenna, the
four-line helical antenna 111 having the four linear conducting
bodies 111a to 111d helically winding on the circumference of the
cylindrical dielectric body 113, the configuration is not limited
thereto. For example, it is allowable to use another circular
polarized antenna, such as a cross dipole antenna, instead of the
helical antenna 111. Moreover, a two-line helical antenna and a
single-line helical antenna can be used instead of the four-line
helical antenna 111.
[0070] According to an aspect of an antenna device of the present
invention, the antenna directivity is increased to a sufficiently
high level.
[0071] All examples and conditional language recited herein are
intended for pedagogical purposes to aid the reader in
understanding the invention and the concepts contributed by the
inventor to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions, nor does the organization of such examples in the
specification relate to a showing of the superiority and
inferiority of the invention. Although the embodiment of the
present invention has been described in detail, it should be
understood that the various changes, substitutions, and alterations
could be made hereto without departing from the spirit and scope of
the invention.
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